Ground fault circuit interrupting (GFCI) devices have recently been developed in ratings suitable for implementation in residential circuits. These GFCI devices typically include, in addition to ground fault protection, overload and short circuit protection. All of these protective features are packaged in a molded case comparable in size to a conventional circuit breaker, such that GFCI devices can be accommodated in existing circuit breaker load centers.
Ground fault protection is afforded by a so-called "module" which energizes a trip solenoid to initiate circuit interruption. To provide adequate protection in terms of preventing personal injury, the GFCI module must respond to high impedance line to ground faults where the fault current is quite small, e.g., 5 to 6 milliamps. Fault currents in excess of 5 to 6 milliamps flowing through the body for even a short duration can produce harmful and even fatal consequences.
A GFCI module basically includes a current sensor, a signal processor and an electronic switch. The current sensor is in the form of a differential current transformer which responds to a current imbalance in the line and neutral conductors of the distribution circuit, as is occasioned by leakage current flowing from the line conductor through a fault to ground and back to the source over a circuit path other than the neutral conductor. The transformer response to this current imbalance or differential is amplified by the signal processor pursuant to triggering the electronic switch, typically a silicon controlled rectifier, and thereby complete an energization circuit for the trip solenoid. With the extremely low signal levels involved, the signal processor must be relatively sophisticated in design and include provisions for rejecting electrical noise in order to provide reasonable immunity to nuisance tripping.
In addition to tripping in the event of a line to ground fault, GFCI devices are required to trip in the event of a low impedance ground fault on the neutral conductor. This is done to guard against the possible desensitizing effect that a neutral ground fault has on the current sensor. That is, should the line and neutral conductors both experience ground faults, some of the current flowing through the lineground fault could return to the source through the neutral-ground fault and the neutral conductor. The current differential seen by the current sensor would not therefore be a true measure of the ground fault current magnitude, and the GFCI device would not trip even though the fault current exceeded the 5-6 milliamp trip level. To also sense a neutral ground fault, the module is further typically equipped with a second transformer which is energized to induce an imbalancing current flow in the neutral conductor of sufficient magnitude to precipitate a ground fault trip function in the event the neutral conductor experiences a ground fault.
It will be appreciated that assembly of the component parts of a GFCI module having all of these requisite capabilities into a compact package of small physical size is an exacting task which contributes significantly to the overall manufacturing cost of GFCI devices, such as GFCI circuit breakers.
It is accordingly an object of the present invention to provide a GFCI device having an improved module incorporated therein.
Another object is to provide a GFCI module designed to facilitate assembly of its component parts into a compact package of small physical size.
Yet another object is to provide a GFCI module of the above character wherein the assemblage of its component parts is maintained without resorting to potting.
A further object is to provide a GFCI module of the above character with improved shielding against RF noise which could precipitate nuisance tripping of the GFCI device.
Other objects of the invention will become apparent from the following detailed description and claims.